U.S. Pat. No. 5,344,463 discloses hair dye compositions and methods utilizing as couplers 2-substituted-1-naphthol compounds of the Formula I, or salts thereof, ##STR2## wherein X is hydrogen or halogen; R is C.sub.1 -C.sub.6 alkyl, hydroxy C.sub.1 -C.sub.6 alkyl, amino C.sub.1 -C.sub.6 alkyl, wherein the amino group is substituted by R.sup.1 and R.sup.2 with R.sup.1 and R.sup.2 being, independently, hydrogen, C.sub.1 -C.sub.6 alkyl, hydroxy C.sub.1 -C.sub.6 alkyl, or R.sup.1 and R.sup.2 taken together with the nitrogen atom to which they are attached form a 5 or 6 membered saturated ring or an olefinic group selected from the group consisting of ethylene, propylene and butylene.
1-naphthol has been used in hair dye compositions for many years. It has proven to have long term storage stability. One skilled in the art would expect the 2-substituted-1-naphthol compounds of U.S. Pat. No. 5,344,463 to likewise have long term storage stability. The present inventors have surprisingly and unexpectedly found that such is not the case.
2-Methyl-1-naphthol decomposes upon standing at room temperature, in air. Within several weeks it changes from a white crystalline compound to a dark liquid. The decomposition was confirmed by HPLC analysis. It might be expected that the instability is related to the electron density of the substrate. The more electron dense the substrate, the greater the instability of the 2-methyl-1-naphthol. However, this was not found to be the case. Several of the 2-methyl-1-naphthols of U.S. Pat. No. 5,344,463 are more storage stable than 2-methyl-1-naphthol even though they are more electron rich. Moreover, 2-methyl-1-trifluoroacetoxynaphthalene is storage unstable, despite the fact that it is more electron deficient than 2-methyl-1-naphthol.
4-Halogenated derivatives of 2-methyl-1-naphthol might be expected to be more storage stable than the parent compound. However, the 4-halogenated derivatives proved an unsatisfactory solution because of their lower solubility in hair dye bases. Moreover, since the halogen weight is lost during dye formation, only a fraction of the weight is useful and dye formation is hindered due to the resultant lower electron density.